Acoustic emission (AE) is a proven non-destructive evaluation (NDE) protocol for monitoring the integrity of structures. A number of NDE methods are available to detect fatigue cracks in structures and each method has one or more significant technical limitations. These NDE methods include visual, tap, ultrasonic, eddy current, and x-ray radiography. Visual inspection is appropriate for checking for surface cracks but inadequate for detecting sub-surface cracks. Eddy current technology can detect cracks but requires a trained NDE technician to properly position eddy current probes and to interpret the test results. X-ray radiography is effective but requires specialized equipment and for safety reasons limits human access to the equipment while the testing is underway. None of these methods are conducive to real time monitoring for providing early warning indications of structural cracks.
Acoustic emission methodology uses an acoustic emission transducer to “listen” for the first signs of the formation of a crack in a structure. Acoustic emission energy is a transient elastic stress wave generated by the rapid release of energy from a localized source within a material. There are many sources of AE that can be recorded for NDE purposes: crack formation and growth, magneto-mechanical realignment or growth of magnetic domains, microstructure changes such as dislocation movement, phase changes, fracture of brittle inclusions or surface films, or even chemical activity resulting from corrosion. Non-destructive evaluation applications of AE basically involve “listening” for sound generated by growing damage in a test piece. Although AE's are generated by the material itself and not by an external source, a stimulus is usually needed to trigger the original AE. These stimuli are sometimes applied by magnetic, thermal, chemical, or mechanical means. When these stimuli are artificially implied for purposes of the test they are sometimes called “active” methods. It is also possible to listen for AE events during in-service use of the structure and this approach is sometimes referred to as a “passive” method.
Traditional AE is widely used and standardized in the oil, petrochemical and rail industries. It has also been used to monitor aircraft structures during ground tests, full-scale fatigue tests, and intermittent flight tests. The traditional implementation equipment is heavy however, often greater than 20 pounds, and large enough to be unsuitable for continuous in-flight use. Traditional AE systems also run a cable from each and every AE transducer all the way back to the centralized computer system. These cable systems require a high bandwidth because of the extensive data transmission required. In addition the traditional implementation transfer an analog signal from the transducer to the central computer. Transferring analog signals over long cable runs increases the likelihood that electrical noise will interfere with the signal. Electrical noise, also called electromagnetic interference, or EMI, is unwanted electrical signals that produce undesirable effects and otherwise disrupt the control system circuits. There is also the potential for radio frequency interference (RFI) from the potential power transfer densities involved due to the proximity of equipment and antenna systems on modern aircraft.
In today's economy aircraft structures must remain in operation for much longer periods of time than originally anticipated. Accordingly the aging effects on these structures are becoming more significant and must be accounted for in the decisions made regarding usage, maintenance, and retirement of the structures. Ideally a real-time in-service monitoring system is needed. Acoustic emission monitoring has great potential for providing in-service monitoring of damage accumulation in this application. AE transducers are small and lightweight and can be permanently mounted. AE data is generated by damage regardless of the size of the individual defect. AE signals created by a flaw propagate through the overall component. If an array of AE transducers is laid out properly, the entire structure can be monitored. Today, however, there is no current real time commercial implementation available due to the aforementioned limitations that current systems are much too large, heavy, and require extensive heavy cabling runs to carry the analog signals back to a central computer. There is thus a long felt need for an implementation of acoustic emission technology that can be used routinely in real time in aircraft structures to either provide warning signals in the cockpit or provide warning diagnostics to maintenance crews at the conclusion of each flight.
U.S. Pat. No. 3,985,024 to Horak is an early AE system developed by Grumman Corporation. It discloses methods for placing AE transducers in ways that enable more accurate predictions of AE source locations. U.S. Pat. No. 4,910,718 to Horn is an AE system specifically designed to locate an AE source in a structural member.
U.S. Pat. No. 6,443,012 to Beardmore discloses a phased array sensing system for an aircraft that includes a central computer and a display system in the cockpit. The matrix arrays are composed of a square or rectangular array of multiple PZT material blocks that are actively pulsed to generate acoustic waves through the structure. The analysis of the AE data is not described other than it is based on imaging technology.
U.S. Pat. Nos. 6,076,405 and 6,014,896, both to Schoess, disclose a remote self-powered AE monitor which has a single AE transducer and still has a centralized computer but eliminates the cabling system by use of a antenna module that sends a continuous stream of data from the acoustic emission transducer to that centralized computer and includes a radio frequency telemetry circuit to supply power to a power storage device with the power storage device positioned on the transducer to provide an inertial load.
The systems described in these patents are either not designed for an array of multiple transducers (U.S. Pat. Nos. 6,076,405 and 6,014,896) or they involve long cable runs to a large centralized computer system. None have been found suitable for commercial applications for aircraft, particularly smaller fighter aircraft, which have little free space for new complex systems. One smaller system is commercially known. The microDiSP from Physical Acoustics Corporation is a smaller portable battery operated acoustic emission system. The chassis of this system though is still 16 inches by 9.5 inches by 2.9 inches high, requires a notebook computer to operate, and does not address the sensor cabling issue outlined above.
What is needed then is a new approach that provides full analysis performance for an array of acoustic transducers that can be implemented in real time on an aircraft while taking up little room and requiring no long cable runs back to a central computer.